1. Field of the Invention
The present invention relates to fuel burners and, more particularly, to an improved burner for reducing the formation of nitric oxides during the combustion of pulverized, liquid or gaseous fuels.
One source of atmospheric pollution is the nitrogen oxides (NO.sub.x) present in the stack emission of fossil fuel fired steam generating units. Nitric oxide (NO) is an invisible, relatively harmless gas. However, as it passes through the vapor generator and comes into contact with oxygen, it reacts to form nitrogen dioxide (NO.sub.2) or other oxides of nitrogen collectively referred to as nitric oxides. Nitrogen dioxide is a yellow-brown gas which, in sufficient concentrations, is toxic to animal and plant life. It is this gas which may create the visible haze at the stack discharge of a vapor generator.
Nitric oxide is formed as a result of the reaction of nitrogen and oxygen and may be thermal nitric oxide and/or fuel nitric oxide. The former occurs from the reaction of the nitrogen and oxygen contained in the air supplied for the combustion of a fossil fuel whereas the latter results from the reaction of the nitrogen contained in the fuel with oxygen in the combustion air.
The rate at which thermal nitric oxide is formed is dependent upon any or a combination of the following variables; (1) flame temperature, (2) residence time of the combustion gases in the high temperature zone and (3) excess oxygen supply. The rate of formation of nitric oxide increases as flame temperature increases. However, the reaction takes time and a mixture of nitrogen and oxygen at a given temperature for a very short time may produce less nitric oxide than the same mixture at a lower temperature, but for a longer period of time. In vapor generators of the type hereunder discussion wherein the combustion of fuel and air may generate flame temperatures in the order of 3,700.degree. F., the time-temperature relationship governing the reaction is such that at flame temperatures below 2,900.degree. F. no appreciable nitric oxide (NO) is produced, whereas above 2,900.degree. F. the rate of reaction increases rapidly.
The rate at which fuel nitric oxide is formed is principally dependent on the oxygen supply in the ignition zone and no appreciable nitric oxide is produced under a reducing atmosphere; that is, a condition where the level of oxygen in the ignition zone is below that required for a complete burning of the fuel.
It is apparent from the foregoing discussion that the formation of thermal nitric oxide can be reduced by reducing flame temperatures in any degree and will be minimized with a flame temperature at or below 2,900.degree. F. and that the formation of fuel nitric oxide will be inhibited by reducing the rate of oxygen introduction to the flame, i.e., air/fuel mixing.
In the U.S., Federal and state regulations are forcing development of fossil fueled combustion equipment capable of reduced NO.sub.x production. Lower NO.sub.x emission requirements apply to pulverized, liquid and gaseous fuels, such as coal, oil and natural gas. While the energy shortages of the early 1970's have contributed to efforts for conservation of oil and gas, utilities in many areas of the country are unable to convert their oil and gas fired vapor generators to coal firing due either to limitations of the existing equipment or due to the increased particulate emissions attendant with coal vapor steam generators. In other situations, the need sometimes arises for a burner having hardware capable of firing all three fuels, though not necessarily more than one of these three fuels at a time. Accordingly, a need exists for equipment capable of achieving reduced NO.sub.x emissions when firing coal, oil and, in particular, natural gas, and which can be retrofitted to existing steam generator units.
2. Description of the Prior Art
Reducing NO.sub.x emissions from fossil-fueled vapor generator units can take several approaches. One approach uses fuels lower in nitrogen content, if such flexibility is available. This only addresses part of the problem, however, and fails to address NO.sub.x production arising out of the combustion process itself. Further, Federal and/or State emission regulations may take the lower fuel bound nitrogen levels into account when setting the standards to be met, and thus set a target level lower than what had to be met with the original fuel(s).
A second approach focuses on cleaning up the NO.sub.x emissions produced by the combustion process itself, taking the nitrogen in the fuel and the efficiencies of the burning of the fuel as given factors in the overall process. One example of this is disclosed in U.S. Pat. No. 4,309,386 to Pirsh, assigned to The Babcock & Wilcox Company. Pirsh discloses a filter house that employs a selective catalytic reduction process for removing NO.sub.x emissions from a flue gas stream while simultaneously filtering out and collecting entrained particulate matter from the stream. An extended treatment of both of the above approaches is beyond the scope and focus of the present application.
The third approach focuses upon the formation of NO.sub.x emissions during the combustion process itself, and is what was referred to earlier as thermal nitric oxide and/or fuel nitric oxide. The combustion process invloves the introduction of a fossil fuel and air into the furnace of the steam generator. Developments have thus focused on the fuel/air introduction equipment, alone, as well as in combination with the furnace of the steam generator.
Krippene, et al (U.S. Pat. No. 3,788,796), also assigned to The Babcock & Wilcox Company, is drawn to an improved pulverized fuel burner apparatus and method for inhibiting the formation of fuel nitric oxide and providing the lower peak flame temperatures required to minimize the formation of thermal nitric oxide. Krippene, et al's burner is known in the art as a dual register burner (DRB) because it employs two dampers or registers for separately apportioning and controlling combustion air flow between inner and outer annular passageways. The inner and outer annular passageways are concentrically placed around a central, tubular pulverized fuel nozzle. The pulverized fuel nozzle conveys a mixture of pulverized fuel and combustion/transport air to the furnace where it is ignited and burned with the rest of the combustion air flow provided by the aforementioned inner and outer annular passageways.
Peterson, et al (U.S. Pat. No. 3,904,349), also assigned to The Babcock & Wilcox Company, is drawn to an improved liquid or gaseous fuel burner apparatus having a central passageway, a first and a second annular passageway, and separate means for apportioning the flow of combustion air among these passageways to achieve complete combustion of the fuel while reducing the formation of nitric oxides. The liquid fuel supplied to and atomized within the burner is sprayed into the circular burner port of the furnace in a pattern substantially symmetrical with the axis of the port. A central fuel tube or nozzle conveys the liquid or gaseous fuel to an atomizing assembly including a sprayer plate located at the outlet end of the fuel tube or nozzle. The central fuel tube or nozzle extends through and out of a guide tube which supports at its distal end a truncated cone air deflecting device, through which the sprayer plate extends, which deflects combustion air conveyed by the central passageway and a portion of the combustion air conveyed by the first annular passageway. Initial burning of the fuel is conducted in a reducing zone by adjusting the quantity of combustion air discharged through the central passageway; air admitted through the first annular passageway causes recirculation of air about the outer periphery of the reducing zone to create a flame stabilizing zone; and finally, the remaining air for complete combustion is discharged through the second annular passageway so as to envelop the reducing and stabilizing zones and eventually mix with the fuel to complete its combustion.
LaRue, et al (U.S. Pat. No. 4,380,202), also assigned to The Babcock & Wilcox Company, is drawn to a mixer for a dual register burner for the combustion of pulverized fuel. Instead of the venturi section and conical end-shaped rod member utilized in the apparatus of Krippene, et al, supra, a deflector and a diffuser having a plug and a shroud member are located within the tubular pulverized fuel nozzle. As a result, flow separation or fuel roping which can occur in the pulverized fuel nozzle is eliminated with minimum pressure loss effect on the primary air/pulverized fuel stream.
As indicated earlier, another development to reduce NO.sub.x formation in the combustion of fossil fuels focuses on the combination/placement of the fuel/air burning equipment with respect to the furnace itself, and is known as two-stage combustion or TSC. TSC involves establishing a lower, air deficient burner zone and an upper/downstream "after-air" or "over-fire-air" zone in the furnace. The amount of air by which the lower burner zone is deficient is injected in the over-fire-air zone downstream to complete the combustion process. In essence, the whole furnace is used as the combustion zone. A more refined version of TSC developed as a result of strict NO.sub.x emission limits in Japan and is known as In-Furnace NO.sub.x Reduction or IFNR. A description of this process is contained in a paper entitled "Advanced In-Furnace NO.sub.x Reduction Systems to Control Emissions" by M. A. Acree and A. D. LaRue, presented to the American Power Conference in Chicago, Illinois on Apr. 22-24, 1985.
Briefly, the IFNR approach, jointly developed by Babcock-Hitachi K. K. and Tokyo Electric Power Company, employs multiple combustion zones in the furnace. The main and lowest zone, the burner zone, utilizes low NO.sub.x burners operated at less than theoretical air levels to reduce the total amount of NO.sub.x produced. The gases and char from this main burner zone pass upwards into a reburning zone, that operates at even lower air levels. Due to the low air levels, the fuel decomposes and forms hydrocarbon radicals that chemically combine to reduce the NO.sub.x directly and which, in turn, further reduce the NO.sub.x present. Upon leaving the reburning zone, since the NO.sub.x levels in the flue gas have been reduced, the balance of the combustion air needed is introduced via overfire air parts in the combustion zone.
In new steam generator construction that applies either TSC or IFNR technology, the furnace volume and height are chosen to accommodate the extended combustion requirements so that the combustion products are completely burned before the flue gas passes across the radiant and/or convective heat transfer tube banks of the vapor generator.
In a retrofit application, however, the furnace volume and height are usually not variable, and the optimum furnace dimensions needed for proper application of TSC or IFNR may not be available. These problems were discussed in a paper entitled "Operating Experiences of Coal Fired Utility Boilers Using Hitachi NO.sub.x Reduction Burners", By T. Narita, F. Koda, T. Masai, S. Morita, and S. Azuhata, presented at the 1987 Joint Symposium on Stationary Combustion NO.sub.x Control, in New Orleans, La., on Mar. 23-26, 1987, sponsored by the U.S. Environmental Protection Agency and the Electric Power Research Institute. As indicated in both the Acree, et al and Narita, et al papers discussed above, generation of low NO.sub.x levels minimizes the amount of NO.sub.x to be destroyed downstream. Improvement of the existing dual register burner (DRB) led to the development of what is known in the art as the Hitachi-NR burner (HTNR) for pulverized coal and the Primary Gas-Dual Register Burner (PG-DRB) for liquid and gaseous fuels.
Morita, et al (U.S. Pat. No. 4,545,307) is drawn to the improved HTNR burner mentioned above. In the prior art DRB (such as Krippene, et al) the pulverized coal stream is supplied with only enough air to transport the coal; consequently, the burner flame at the burner throat entrance to the furnace formed a good reducing atmosphere. The balance of the combustion air, called secondary and tertiary air, came to the burner throat via the inner and outer annular passageways, respectively, and was to mix downstream of the central, reducing atmosphere burner flame. Too early mixing, however, of the secondary/tertiary air and the reducing atmosphere burner flame made maintaining the latter difficult.
Morita, et al modified the DRB for coal firing by attaching a bluff body at the outlet of the pulverized coal pipe, shaped as a ring-form dish having a hole therethrough for passing the pulverized coal/air mixture into the furnace. A portion or apron of the bluff body protrudes into the inside diameter of the pulverized coal pipe to enhance ignitability at the exit thereof, while the outside diameter of the bluff body extends outside of the pulverized coal pipe partially into the secondary air (inner annular) passageway. In addition, an outward guide sleeve is provided, between the secondary air (inner annular) passageway and the tertiary (outer annular) passageway to dispense the tertiary air outwards beyond the central pulverized coal flame, later combining downstream to complete the combustion process. The bluff body creates an eddy flow in the pulverized coal/air stream supplied by the pulverized coal pipe which prevents it from diffusing in an outward manner towards the secondary air stream.
Other development work on improved burners for coal firing has occurred, and is presented in a paper entitled "Development Status of B&W's Second Generation Low NO.sub.x Burner--the XCL Burner", by A. D. LaRue, M. A. Acree and C. C. Masser, presented at the 1987 Joint Symposium on Stationary Combustion NO.sub.x control, in New Orleans, La., on Mar. 23-27, 1987, sponsored by the U.S. Environmental Protection Agency and the Electric Power Research Institute. The XCL burner design disclosed therein while using criteria from the HTNR and DRB burners discussed earlier, was developed for coal firing only.
The Primary Gas-Dual Register Burner (PG-DRB) for oil and gas firing, is a DRB modified to include a recirculated gas annulus which surrounds a primary air zone that houses the oil atomizer, and is disclosed in the Acree, et al reference mentioned above. It should be noted that, in this context, the term "recirculated gas" refers to flue gas, rather than fuel gas. The source of the recirculated gas would be from a point somewhere downstream of the last heat transfer surface in the steam generator, for example at the economizer outlet. The recirculated gas shields the base of the oil flame to reduce oxygen availability in the flame core; mixing of recirculated gas with the rest of the combustion air results in all of the combustion air having a lower oxygen content to further suppress NO.sub.x production.
For gas firing, as will be seen by a review of FIG. 4 of the Acree, et al reference, supra, and by referring to FIG. 1 of the present application which shows a slightly modified version of the PG-DRB in schematic form, the gas elements 1 of the PG-DRB are placed in the tertiary air passageways 2 which encircle, successively, the oil atomizer 3, the primary air zone 4, the primary gas zone 5, and the secondary air zone 6. While each gas element 1 has at the outlet end thereof shields 7 which protect the gas outlet nozzle 8 on each gas element 1, it is clearly seen that each gas outlet nozzle 8 is continuously swept by the combustion air flow passing out into the furnace 9 through the tertiary air passageways 2. This arrangement prevents the establishment of any fuel rich/low air reducing zone in the vicinity of each gas outlet nozzle 8 that is crucial for low NO.sub.x emissions. In addition, some applications will also prohibit the use of the PG-DRB scheme, and yet the need for reducing NO.sub.x emissions on gas firing will remain.
Accordingly, it has become desirable to develop an improved burner apparatus capable of separately firing pulverized, liquid or gaseous fuels and which can achieve reduced NO.sub.x emissions on each of these fuels.